The present invention discloses novel stabilizers used to extend the useful-life of formulations containing photoacid precursors. These stabilizers are useful for extending formulation shelf-life and for extending the in-use life in cases where the formulation is exposed to low initiating light levels such as room lighting or extraneous light from an exposure source. In addition, this invention discloses novel methods for comprising the stabilizers in the formulation such that the stabilizer concentration remains substantially constant and such that the photohardening speed of the composition is essentially unaffected.
Photoacid precursors are often used in compositions to initiate polymerization, to cause de-polymerization, and to generate color. Many of the photoacid precursors have been found to be thermally or hydrolytically unstable. Some of the photoacid precursors may be acceptably thermally and hydrolytically stable, however, there may be residual acid in the photoacid precursor product, or there may be some acid generation due to absorption of light from the room or extraneous light from the exposure process. In some compositions, for example epoxy based compositions, a small amount of this acid will initiate the polymerization and eventually lead to a gellation of the polymer or actual hardening of the polymer.
Various stabilizers have been proposed. For example, U.S. Pat. No. 3,721,616 by Watt proposes the use of various nitrile compounds such as acetonitrile which stabilized some epoxy compositions containing chlorobenzenediazonium hexaflorophosphate for several days. The samples without the nitrile compounds formed a gel within four days. Nitrile compounds may be useful for some systems, however, now that most of them are considered suspect carcinogens, it would usually be preferable to avoid these stabilizers especially in processes that involve significant skin contact. Within this patent Watt teaches that it is undesirable to have a stabilizer that forms undissolved solid particles or immiscible liquid globules. This teaches away from the instant invention wherein undissolved solid particles are preferred and significantly advantageous. Also in U.S. Pat. No. 3,721,617 Watt proposes the use of cyclic amide gelation inhibitors for epoxy systems. While these may be useful in some systems, it has been found that some of these cyclic amides, for example polyvinylpyrrolidone, significantly inhibit the polymerization of epoxy systems.
In general, for example in photohardenable epoxy systems, it is the acid that is produced which does not cause significant gellation or hardening of the epoxy that creates the greatest problem of composition instability. This acid is mobile within the composition and is capable of initiating many epoxy reactions throughout the formulation. In regions where the epoxy is gelled or hardened, the acid is significantly immobilized and therefore has a much more limited opportunity to initiate further polymerization other than in the exposed region.
The necessity for a stabilizer is perhaps most apparent in, for example, Solid Imaging systems using photohardenable liquid epoxy compositions. In the Solid Imaging process, the liquid epoxies may be stored for long periods of time. Typically, the liquid epoxy is utilized in deep vats, for example 500 mm deep vats. Even though the photoacid precursor and or a sensitizer for the precursor may not be particularly absorbing at room lighting wavelengths, the depth of the vat provides a significant absorption cross-section such that the light that passes into the vat has ample opportunity to cause some actinic effect. While yellow or red room lights may be recommended to avoid the effects of room light exposure, some users of Solid Imaging systems are reticent to convert to these lighting conditions.
Perhaps a greater cause of generation of acid from photoacid precursors in the Solid Imaging process is due to the process itself. In most cases, imagewise exposure of the epoxy composition is performed with a focused laser beam. This beam has a Gaussian spread which is capable of creating significant exposure outside the nominal 1/e.sup.2 spot diameter. At some point outside this diameter, the exposure, while still capable of generating acid, does not generate a large enough concentration of acid to polymerize the epoxy to the point of insolubilization. This leaves the acid free to migrate throughout the vat and initiate polymerization which leads to a viscosity increase of the formulation. A similar situation occurs in cantilevered or bridging regions where the exposure through the layer being imaged and hardened reaches a point that the amount of exposure is no longer capable of producing enough acid to polymerize the epoxy such that the acid becomes locked in. Another system source of free acid occurs, for example, when the beam is moved from one image region to another. If the exposure system uses an acousto-optic modulator, the beam irradiance is often reduced to a lower level, for example 1/25'th or 1/1000'th the imaging irradiance, and the beam is often moved very quickly from one image region to another. However, this reduced exposure is still capable of generating acid even if it is not capable of insolubilizing the epoxy composition along the path. Further system sources of such low exposures occur from reflections within the chamber caused by, for example, reflections from the surface of the liquid, reflections from various beam sensors, and scattered light from mirrors and optics.
Many bases are capable of neutralizing the acid generated thermally, hydrolytically, or by light. However, some bases are strong enough to cause reaction or polymerization and therefore are not useful stabilizers. The concentration of the stabilizing base is important. If the base concentration is too high, any acid generated by exposure of the photoacid precursor will become neutralized by the base. Therefore low concentrations of base are preferred in order to provide minimal effect on the desired cure. However, in applications such as for example Solid Imaging, where a reservoir of photohardenable composition is maintained and the material is exposed by room light and extraneous system light at variable rates, there is the possibility that the stabilizer will eventually become consumed and the stability of the photohardenable composition will be lost.